Strip flattening



Sept. 8,1970 J K. T. LAW SON ET AL 3,527,078

STRIP FLATTENING Filed Aug. 12, 1968 9 Sheets-Sheet 1 FIG].

M25 moat/u XMM/ 7% 5r .Mdwm' Jzflii /924.20%

Sept. 8, 1970 T LAWSON ET AL 3,527,078

' STRIP FLATTENING Filed Aug. 12, 1968 9 Sheets-Sheet z Sept. 8, 1970Filed Aug. 12, 1968 Ty Ty=ylld St/QSS 50.

NUMBER OF BEND/N6 ROLLS -3 TUTAL ELONGAT/ON OF STRIP 0.5% STRAINOFMATER/AL AT YIELD STRESS IS .0015.

K. T. LAwsofi ET AL 3,527,078

Sept. 8, 1970 STRIP FLATTENING 9 Sheets-Sheet 5 Filed Aug. 12, 1968Sept. 8, 1970 K. T. LAWSON ET AL 3,527,078

STRIP FLATTENING Filed; Aug. 12, 1968 9 Sheets-Sheet 6 Sept. 8, 1970 KLAWSON ET AL 3,527,078

STRIP FLATTENING Filed Aug. 12, 1968 9 Sheets-Sheet F Sept. 8, 1970 wsoET AL 3,527,078

STRIP FLATTENING Filed Aug. 12, 1968 9 Sheets-Sheet 8 Sept. 8, 1970 K.T. LAWSON E AL 3,527,073

STRIP FLATTENING Filed Aug. 12, 1968 9 Sheets-Sheet 9 United StatesPatent 3,527,078 STRIP FLATTENING Kenneth Thomas Lawson, Marten, and.lerzy Franciszek Nofer, Middlesbrough, England, assignors to Head,Wrightson and Company, Limited, Thornaby-on-Tees, Yorkshire, England, acorporation of the United Kingdom Filed Aug. 12, 1968, Ser. No. 752,009Int. Cl. B21d 1/02 U5. (:1. 72--l60 Claims ABSTRACT OF THE DISCLOSUREContinuous metal strip is flattened by driving it at two spacedlocations at speeds difiering by a predetermined proportion, and betweenthese locations the strip is subjected to the action of a roll system inwhich the strip enwraps a floating roll so as to cause the roll to pinchthe strip against further rolls at the points of which the strips wrapsonto and unwraps from the floating roll. This may be followed by afurther stretch levelling operation. Apparatus is provided comprisingfirst and second sets of bridle rolls and between them in one case apair of floating rolls co-acting with a central roll and the innermostrolls of the bridles and in a second case a pair of floating rollscoaching with each other and a pair of large diameter backing rolls. Inan alternative form of apparatus a further downstream bridle isprovided, each bridle being driven at a speed higher than the previousone by a selected proportion, so that the strip is subjected to afurther stretching operation either with or without the action ofbending rolls. In each case the floating roll or rolls have a diameterat least 25 times the thickness of the strip to be heated and a radiussmall enough for the strip to be subject to plastic deformation whenbent to this radius under the influence of the tension to which it issubjected.

The invention relates to the flattening of continuous metal strip,particularly for the purpose of removing such faults in strip shape aslong or wavy edges or long middles as may be produced during previousreduction in a rolling mill and for relieving any residual forces withinthe strip which may tend to cause unevenness.

In order to eliminate such defects, successive portions of the strip aresubjected to forces which cause plastic deformation of the metal, thusremoving residual forces which tend to cause unevenness in the strip aswell as eliminating any actual dimensional unevenness.

In the past two main approaches have been adopted in deforming thestrip; firstly, stretch levelling, which entails subjecting successiveportions of the strip to a longitudinal force sufficient to stretch thestrip beyond its yield point. This often gives satisfactory results, butit will be appreciated that, particularly with strip in large widths orheavy gauges, the forces required to be applied are very high, requiringextremely robust equipment, and also there are problems associated withretaining adequate control over the degree of elongation occurring inthe unsupported strip during stretching, which may therefore be unevenafter stretching. Moreover, where appreciable elongation is required toflatten the strip, unsightly marks known as stretcher strains may beproduced.

The other main method is roller levelling, which entails passing thestrip between two staggered interlaced series of rollers, whereby it isflexed sharply first in one direction then the other. Conventionalroller levellers are generally considered less effective than stretchlevellers.

Various attempts have been made to improve the performance of levellersby combining the stretching and "ice bending principles, and here againtwo main approaches have been adopted. One is merely to apply increasedlongitudinal tension to the strip as it passes through a rollerleveller. Although this tension may not be suificient in itself tostretch the strip, it makes the strip hug the bending rolls in theleveller more tightly, thus assisting to ensure plastic deformation ofthe strip during bending providing the bending rolls are a suitablediameter. The problems of this approach are that the tensions requiredto ensure that the strip accurately follows the contours of the bendingrolls produce increased forces in the bending rolls and make it moredifficult to keep deflection of the rolls within reasonable bounds.These problems are aggra vated when it is desired that the levellersshould handle Wide strip, particularly when the same machine may berequired to handle a considerable range of thicknesses.

The other approach has been to reduce the size of the bending roll towell below that which will ensure plastic deformation of strip of thesmallest gauge to be treated, but the use of such small rolls presentsits own problems in that it is difiicult to provide them with adequatesupport and also they are liable to rapid wear and fracture.

We have found that a soution both to the problem of supporting the rollsand of making the strip hug the bending rolls is provided by usingparticular apparatus comprising inter alia a roll arrangement similar inprinciple to one which has previously been proposed in US. Pat. No.2,291,361 for the implementation of a method of hardening continuoussoft or annealed ferrous material. The method utilises the simultaneousapplication of tension, bending over a small diameter bend roll, androlling of the strip under pressure of the small diameter bend rollwhich is pressed against a pair of backing rolls by the tension in thestrip. The arrangement is such that a loop in the strip passes through anarrow gap between two backing rolls of substantial diameter, a verysmall diameter roll or rod being inserted in the loop. On application oftension to the strip, the small bending roll is pulled into a gapbetween the backing rolls, and the strip is constrained to follow a pathover one backing roll round the bending roll and over the second backingroll. The bending roll is held in place by the strip itself, and thebend roll and strip are very adequately supported by the large diameterbacking rolls which are sufliciently stiff to render roll deflectionunder load quite negligible. This arrangement makes it possible toutilise a bend roll as small as Ms" diameter, which would be quiteimpracticable with conventional backing rolls. However, there stillremains the problem of wear or breakage of the bending roll, andmoreover, when the arrangement is used for stretching continuous stripmaterial it has certain limitations. The buildup of tension in the stripappears to depend mainly on the amount of work expended in bending androlling the strip on the small diameter bending roll and on thefrictional drag in roll bearings, etc., so that the amount of elongationto which the strip is subjected varies according to the thickness andmaterial properties of the strip being handled and cannot be adjustedand controlled by means of this apparatus in its basic form. Moreover,using the ratios of strip thickness to bending roll diameter hithertoproposed, the degree of reduction and elongation of the strip tends tobe excessive for levelling purposes, although it may be useful for stripreduction. Finally, the patentee himself admits, and in fact requiresthat surface fractures be formed in the strip. It is believed that he isreferring to the phenomenon known as Liiders lines, a form of surfacedefect that it is in most instances extremely important to avoid inlevelling operations. This apparatus is thus unacceptable for normallevelling purposes.

A similar roll system has also been proposed in US.

Pats. 2,332,796 and 3,238,756 for the purpose of obtaining stripreduction. Whilst many levelling operations are accompanied by somereduction of the strip, this is quite incidental to the main purpose ofthe operation, i.e. levelling. These patents contain no suggestion thatthe apparatus they disclose can, suitably modified, form the basis of anexceptionally efficient strip levelling system, and indeed, even werethe theoretical considerations involved in strip reduction and striplevelling not quite different, thus contraindicating the applicabilityof apparatus useful in the one art to the other art, the disclosure ofPat. No. 2,291,361 would suggest that they were useless for thispurpose. Thus although British patent specification No. 1,002,936 in theform in which it was originally published showed strip flatteningapparatus consisting of several of the roll arrangements of No.2,291,361 in series, it is found in practice that the arrangement isinoperative for a number of reasons, including a tendency to damage thestrip. The above British specification was later amended to exclude thismatter.

The object of the invention is to provide a machine capable offlattening at high speed continuous strip over a wide range of materialproperties, thicknesses and widths and faulty shape, and in whichprovision can readily be made for easy threading of the strip.

Surprisingly, we have found however that improved stretch levellingapparatus is produced if a similar bending roll system to that describedabove in which the diameter of the bending rolls has a particularrelationship discussed below to the thickness of the strip is flanked bymeans positively driving the strip, the driving means subsequent to thebending roll system being adapted to drive the strip at a predeterminedfaster rate than the means preceding the bending roll system, thepercentage increase in speed being equal to the degree of elongation inthe strip which is found necessary to produce the desired levellingeffect. The bending roll system must however be such as to permit easythreading of the system by the strip to be treated, this threadingoperation having previously provided a considerable impediment to theuse of roll systems of the type discussed.

A complex system of stresses is developed in the strip in the region ofthe first and third rolls of the three roll system owing to the combinedaction of longitudinal tension, bending around the bending rolls androlling between the bending rolls and adjacent rolls and high stressesin excess of the elastic limit can be developed locally in the strip sothat permanent deformation takes place. The net result is that strip canbe made to undergo permanent elongation even though the overalllongitudinal tension in the strip is itself insufficient to causepermanent extension. In fact the machine can be so designed that theoverall longitudinal tension applied by the bridle rolls is only a smallfraction of that needed to cause general yielding and permanentextension in the strip.

Exemplary embodiments of the invention are described with reference tothe acompanying drawings, in which:

FIG. 1 is a side elevation of a preferred embodiment of a leveller inaccordance with the invention,

FIG. 2 is a plan view of the same leveller,

FIG. 3 is a vertical longitudinal section,

FIG. 4 is a graph showing the influence of the ratio of bending rolls,diameter to strip thickness on the performance of roller levellers,

FIG. 5 is a plan view showing an alternative form of transmission,

FIGS. 6 and 7 are diagrammatic cross sections of modifications of theleveller showing alternative methods of providing for threading of theleveller,

FIG. 8 shows diagrammatically an alternative embodiment of the leveller,

FIG. 9 shows diagrammatically an alternative form of roll system for usein the invention,

FIG. 10 shows diagrammatically a further embodiment of the invention,

FIG. 10a is a diagrammatic detail showing a development of theembodiment of FIG. 10, and

FIG. 11 shows diagrammatically an alternative form of apparatus forimplementing the method of the invention.

Referring to FIGS. l3, rolls 14 and S8 are drag and pulling bridle rollsrespectively for driving and tensioning the strip, and constitute firstand second positive drive means respectively. They are mounted inantifriction bearings and driven via a transmission to be describedbelow from a single motor 25.

On leaving roll 4 the strip under tension passes between the roll 4 anda roll 10 and forms a loop around a bending roll 9 before passing backbetween rolls 4 and 10, over roll 10, and between rolls 10 and 5 to forma loop embracing a bending roll 11, the tension in the strip supportingrolls 9 and 11 respectively adjacent to rolls 4 and 10 and rolls 10 and5, the strip being nipped between the rolls. Rolls 9 and 11 areconstrained against axial movement in order to prevent them moving outof the machine laterally, but they are free to take up a radial positionas dictated by the geometry of the strip and rolls 4, 5 and 10. Therolling load on the strip applied between rolls 4 and 9, 9 and 10, 10and 11, 11 and 5 depends upon the magnitude of strip tension and therelative positions of the rolls. Roll 10 runs in journal bearings and ismounted in bearing housings 1% which can slide along vertical guides10c. The position of roll 10 is adjustable by means of a worm drive andscrew jack 16 to control over a limited range the rolling load on thestrip. Rolling of the strip is advantageous in that it improves surfacefinish and tends to prevent formation of unsightly stretcher strainmarkings. However, the rolling forces applied are quite small incomparison to rolling forces applied in conventional skin pass rollingmills and the elongation of the strip is achieved almost entirely bytension and bending. Indeed the chief advantage of the roll arrangementemployed is that it enables the bending rolls to be adequately supportedin such a way that deflections are greatly minimised and friction lossesin bearings are minimised, whilst proper contact of the strip with therolls is ensured. Rolls 9-11 can be retracted to the positions 9a-11ashown in broken lines to simplify threading of the leading edge of stripmaterial through the machine.

The bridle rolls 1 to 8 are driven via shafts 28 and 29 respectivelythrough two sets of gears in gearbox 21. A variable speed gearbox 22 andplanetary gearbox 23 form the means for varying and controlling theamount of elongation applied to the strip by adjusting the relativespeed of bridles 1, 2, 3 and 4 to bridle rolls 5, 6, 7 and 8. Fromgearbox 24, output shaft 26 drives the input gear in planetary gearbox23 and via variable speed gearbox 22, the casing carrying the planetgears of planetary gearbox 23. The speed of shaft 28 relative to thespeed of shaft 29 is therefore adjustable by selecting different gearratios on the variable speed gearbox 22 and this enables the relativespeed of bridle rolls 1, 2, 3 and 4 to 5, 6, 7 and 8 to be adjusted.

For the purpose of threading the leading end of a new coil of materialthrough the stretch levelling machine, roll 10 is lowered by means ofthe jack 16 into position 10a (see FIG. 1) and bending rolls 9 and 11take up positions 9a and 11a on supports 30. FIG. 3 shows cylindricalguides 31 around the bridle rolls and also rubber covered pinch rolls 33which are pneumatically operated. Strip is fed between bridle roll 1 andpinch roll 12 and the leading edge is guided around the drag bridlerolls by guide plates 31. As the leading edge advances the pinch rolls33 are operated to press the strip against the bridle rolls; slowrotation of the bridle rolls feeds the strip forward and through themachine. The strip follows a path around guides 32 is deflected by plate34 and is then threaded around the exit bridles 5 to 8.

After threading, the threading pinch rolls 33 are retracted anddeflecting plate 34 removed and roll 10 is moved to its operatingposition taking with it the loop of strip formed in guides 32 and alsorolls 9 and 11. The clutch and brake fitted into the drive to the dragbridles can then be operated to enable rolls to 8 to remain stationarywhilst rolls 1 to 4 are reversed until the strip is pulled tight aroundbending rolls 9 and 11 and throughout the machine. The clutch and brakeare then released and the machine is set into operation in the normalway.

Principal data for a typical stretch levelling machine is given below asan example.

Material Steel strip (21 tons/in. at

2% proof stress).

Maximum strip width 72".

Maximum strip thickness 0.078".

Operating speed 600 f.p.m.

Bridle roll diameter 30".

Bending roll diameter 4".

Number of bending rolls 3.

Main drive motor 850 HP.

Face width of rolls 78".

Strip thickness (inches) 0.015; 0.032; 0.048; 0.078.

Percent elongation 2.0; 1.9; 1.6; 1.1.

It will be readily appreciated that the invention described herein isnot limited to the above dimensions and in fact bridle rolls will oftenvary between 20" to 48" diameter, bending rolls may vary within therange 1" to 4", whilst to increase stiffness and support, the centreroll in FIG. 1 may vary in the range 4" to 12", and even these valuesmay be exceeded in exceptional circumstances. The effect of roll 10 asan additional bending roll diminishes as its diameter increases but thiscan be allowed for in design.

FIG. 4 presents typical data for use in design of stretch levellersaccording to the invention and deals with a particular case ofstretching in apparatus similar to that described above using threebending rolls 9, 10, 11 to produce a permanent elongation in the stripof /2%, the yield strain of the strip material being 0.15%. This showshow the tensile stress in the strip varies with the ratio t/D (stripthickness to bending roll diameter ratio). (Tensile stress (T or T isdefined as the tension force in the strip immediately before or afterthe bending rolls divided by the cross-sectional area of the strip andexpressed as a percentage of the yield stress for the strip material.)

Two points are significant:

(a) Strip tension increases rapidly as t/D decreases below the value of0.01. This means that the loads on the machine become considerable fort/D values below 0.01 and the general size and number of bridles anddriving gears, etc., may be increased appreciably thus increasing thecost of the machine. Clearly it is an advantage for t/ D to be greaterthan 0.01.

(b) The difference between front and back tension in the strip increasesrapidly for t/D values greater than about 0.02 and in fact the backtension becomes negligibly small at higher t/D values. Now thedifference in front and back tensions represents the force needed todrag the strip through the bending rolls in the absence of any tensionexerted by the drag bridle and this force is a function of the work donein bending the strip. As t/D increases more and more work is done inbending the strip until the strip tension necessary to drag the stripthrough becomes so great that the desired elongation of the strip isexceeded. Thus a machine with too many undriven bending rolls or withbending rolls of too small a diameter could only produce elongationsbeyond a certain range. To be able to control strip elongation within auseful range of say up to 2% it is essential not to exceed a certain i/Dratio and not to exceed a certain number of undriven bending rolls. Incertain cases, elongations up to 5% may be possible.

Our experience indicates a preferred range of values for 6 t/D liesbetween 0.01 and 0.02, but of course we do not restrict ourselves tothis range. Most machines have to deal with a wide range of requiredelongations and strip thicknesses and to avoid having to replace bendingrolls during operation in order to adjust t/D values for each stripthickness, etc., it may be better to design a machine with a wider rangeof t/ D values. It will be seen from the above example, for instance,that t/D values vary from 0.02 for 0.078" strip with 4 bending rolls to0.00375 for 0.015" strip with 4" bending rolls and this is quiteacceptable. However, to maintain control of elongation and to achieve apractical robust machine it is essential to adopt values for t/D whichare as small as possible and we would always try to maintain t/D withinthe range 0 to 0.02 and would certainly never exceed t/D=0.04, i.e. thediameter of the floating rolls would be at least 25 times the thicknessof the strip.

A machine has been proposed using a bending roll captive within a loopin the strip but it relies in the only empirical example given on dragtension produced by the bending of 0.0107 strip over a small diameter)bending roll (t/D=0.08) in order to achieve hardening of the strip, andas our own work shows this will not give adequate control of stripelongation, and there appears to be no means of varying and controllingstrip elongation in order to deal with various strip materials withdifferent material properties and thicknesses, etc. The presentinvention overcomes these drawbacks.

The above figures for the ratio r/D, although discussed in relation to athree roll bending roll assembly (two floating rolls and one fixed roll,all of approximately equal diameters) apply regardless of the actualnumber of bending rolls employed, and, in the range being considered,the actual arrangement of the bending rolls. The use of differingnumbers of rolls or rolls of differing diameters merely affects thevalue of the overall tension applied to the strip in order to achieve agiven elongation. A small number of bending rolls involves highertensile forces in the strip, whereas a large number would enable theseforces to be reduced. There is for any particular application an optimumnumber of rolls to give minimum cost of machine and maintenance.

Various modifications and additions may be made to the apparatusdescribed.

It may be useful to incorporate a simple unbacked roller levellerimmediately after the leveller of the invention. The roller leveller maybe a separate machine or it may be part of the basic stretch levellingmachine. Strip which has had long edges, etc., removed by stretchlevelling may still have longitudinal curvature when it leaves themachine. If some strip tension is maintained when the strip leaves thetension levelling machine then the strip will be held straightlongitudinally so that longitudinal curvature may not be apparent.However due to the effect of lateral contractions or expansions-thePoisson ratio effect-the strip maybe observed to have lateral curvature,i.e. it will be curved transversely and not longitudi nally. It may notbe necessary to remove this curvature if the strip is to be immediatelyre-coiled. However if the strip were not to be re-coiled, for instanceif it were to be cut into sheets, then it would be essential tostraighten the strip and a roller leveller would be necessary. Such aleveller could be a simple 3 or 5 roll roller leveller, without backingrolls.

FIG. 5 shows an arrangement of machine which has a motor drive(preferably a DC). speed controlled motor) to the pulling bridle rolls 5to 8 and a drag generator 170 braking the drag bridle rolls 1 to 4.

Yet another version of the stretch levelling machine has an auxiliaryDC. motor drive coupled to the casing of the planetary gearbox 23 (seeFIG. 2). This motor is con trolled to apply constant torque onpre-determined values and its function is to relieve the load on thevariable speed gearbox 22.

As another variation, the auxiliary D.C. motor can be used to drive thecasing of the gearbox 23 and the variable speed gearbox 22 can beomitted entirely. In this case the DC. motor would have a speed controlso that speed could be adjusted to pre-determined values in order tocontrol the amount of elongation on the strip. This particulararrangement has the advantage that the auxiliary motor can be operatedwhen the main drive is stationary to drive the drag bridles whilst thepulling bridles remain stationary. This gives a means for tensioning thestrip in the machine immediately after threading and prior to normalrunning.

Referring now to FIGS. 6-7, two alternative ways of providing forthreading of the leveller are described, reference numerals being thesame where applicable as those in the embodiments already described.

In both figures, the three roll system comprising rolls 9, 10, 11 iscarried by a mounting block 100 in which the rolls are movable betweenthe position shown in broken lines and their normal working positionsshown in solid lines. The block 100 itself is movable between a workingposition shown in solid lines and a loading position shown in brokenlines, whilst in FIG. 6 the entire series 1, 2, 3, 4 and 5, 6, 7, 8 ofbridle rolls are displaceable between a working position shown in solidlines and a loading position shown in broken lines: in FIG. 7 only twoadditional rolls 15 and 16 are so displaceable.

To load the leveller in either case, the various movable parts referredto are moved to their position shown in broken lines, thus defining astraight path 101 for the strip which is thus readily threaded throughthe three roll system. The block 100 is then returned to its workingposition, the roll 10 pushed back through the gap between rolls 9 and 11taking the strip with it by means of a jack 102 (see FIG. 6) whichpositively maintains this roll in its working position, the bridle rollsare returned to their working positions and the rolls 9 and 11 drawninto their working positions by the application of tension to the strip.

The leveller may be modified by the provision of additional three rollsystems between the first and second series of bridle rolls, the systembeing separated by additional bridle or deflector rolls. Such a modifiedleveller is shown in FIG. 8, wherein bridle rolls 1-8, and pinch rolls12, 13 are provided as in the embodiment of FIG. 1. However, two sets ofrolls 9, 10, 11 are provided, separated by an idle backing roll 200. Theadvantage of this arrangement is that reduction of the strip isdistributed between a larger number of individual forming operations,and the danger of defects such as Liiders lines appearing is stillfurther reduced; but on the other hand the amount of rolling to whichthe strip is subject is increased, and this may cause a degree ofhardening of the metal which is excessive for some purposes.

In other cases it will be found that eflicient operation may be achievedusing only a single floating roll 201, as illustrated in FIG. 11. Thesingle floating roll is supported by two large diameter idle backingrolls 202, 203, and the strip driven by sets of bridle rolls 14, 8 as inprevious embodiments.

The idle rolls 202, 203 are journalled in a frame so that they are, whenthe apparatus is in use, separated by a gap less than the diameter ofroll 201, the apparatus defining a path for the strip which passesaround roll 202, through the gap, around the roll 201 which has no fixedjournals, back through the gap and around roll 203, so that when tensionis applied to the strip roll 201 is drawn into the gap and pinches thestrip against rolls 202 and 203 at the points where the strip is bendingto conform to the contour of roll 201. By making the journals of roll202 and roll 203 displaceable relative to each other, the use of rolls201 having different diameters is readily allowed for, and thepenetration of the roll 201 into the 8 gap can also be varied so as toalter the contribution the rolling action generated makes to thereduction and consequent elongation of the strip. Moreover the rolls 202and 203 can be separated to permit roll 201 and its surrounding loop tobe passed between them during threading of the machine.

The basic three roll arrangement which has been utilized in the variouspreviously described embodiments has certain inherent disadvantages.Firstly, action on the strip takes place at the two points at which itis pinched between the work and backing rolls, with the result thatslippage occurs between the backing and work rolls and the strip atthese points. This problem is discussed in US. patent specification No.2,332,793 (Hume). This specification proposes various solutions to thisproblem, but none overcome the second disadvantage of the arrangementwhich is that the treatment of the strip is asymmetrical in that oneside of the strip is subjected to the action of the large diameterbacking rolls whilst the other side is subjected to the action of thesmall diameter work roll. The result is that the tWo sides of the stripare likely to have different finishes and degrees of surface hardnesswhich is often undesirable. An alternative arrangement which overcomesthese disadvantages and which may be substituted in the embodimentsdescribed for the three roll system is illustrated diagrammatically inFIG. 9, showing a roll arrangement for incorporation in strip processingapparatus comprising two relatively large diameter backing rolls 301,302 separated by a gap of width less than the sum of the diameters oftwo relatively small diameter floating rolls 303, 304, the rollarrangement defining a path for strip S passing in a first directionover the first backing roll 301 and through the gap, around the firstfloating roll 303 in the opposite direction, through the gap and aroundthe second floating roll 304 in said first direction, and through thegap and over the second backing roll 302 in said opposite direction. Apair of steady rolls 305 may be provided on either side of the gap tocontact the floating rolls via the strip so as to stabilize theirposition within the gap.

The advantages of this arrangement are firstly that most of the rollingaction on the strip takes place at the nip of the two floating workrolls, with the result that problems due to slippage between the workrolls, the strip and the backing rolls are much reduced. Secondly, bothsides of the strip will be treated directly by the smaller diameterrolls so that both sides of the strip will have identical finishes onleaving the device.

A further embodiment of the invention is shown in FIG. 10. It is foundthat strip which has been subjected to a moderate degree of stretchingin apparatus according to any one of the previously describedembodiments may then be subjected to a further stage of stretching by amore conventional form of stretch levelling Without the formation ofstretcher strains. It is found that the forms of apparatus alreadydescribed above are often not ideal for applications in which anelongation of the strip exceeding 2% is required, due to speeddifferentials between the various rolls carrying out combined bendingand rolling operations becoming excessive and in some instances causingsurface marking of the strip and rapid roll wear due to slippage.Moreover, with large elongations, and in some cases even with lowerelonga tions, the amount of rolling received by the strip may beexcessive, leading to an undesirably high degree of hardening of thestrip.

In the present case, the stretching is carried out in two stages, inonly the first of which is the strip subject to rolling by suitableproportioning of the degree of stretch achieved in each stage, and it ispossible to obtain much higher elongation of the strip and thereforemore eflicient elimination of flatness defects without subjecting thestrip to excessive hardening or risking the introduction of surfacedefects due to roll slippage or the formation of Liiders lines.

To this end, the apparatus comprises a first set of bridle rolls 401,402, a second set of bridle rolls 403, 404 and a third set of bridlerolls 405, 406, 407, 408. Either all rolls are driven, or it may bedesirable to leave rolls 402 and 403 undriven, but in any event thedrive to the rolls is contrived so that rolls 405-408 run faster by apredetermined and preferably adjustable margin than rolls 403 and 404,and these latter rolls in turn run at a speed higher by a predeterminedand preferably adjustable margin than rolls 401 and 402.

Rolls 402 and 403 are separated by a gap narrower than a small diameterfloating roll 410, which unlike the bridle rolls is of such a diameterthat the strip during processing will suffer plastic strain on beingbent to conform to its periphery. It is preferred however that the ratioof strip thickness to roll diameter should not exceed 0.04. The roll 410is trapped in a loop in the strip passing between rolls 402 and 403, andduring operation is fully floating, being supported solely by the strip.The tension set up by the speed differential between rolls 401 and 402and rolls 403 and 404 draws the rolls 410 tightly into the gap betweenrolls 402 and 403, causing the strip to conform to roll 410, and roll410 to press the strip against rolls 402 and 403. Thus at the nipsbetween rolls 402 and 403 and roll 410 the strip is both plasticallybent and rolled.

The strip may either pass directly from roll 404 to roll 405, as shownin FIG. 10, being stretched solely by the tension set up by the speeddifferential between the second and third sets of bridle rolls, or, asshown in FIG. 10a, it may be flexed first one way, then the other, overa series of small diameter bending rolls 411, in the manner describedfor example in British patent specifications 655,444 and 758,420. Theintroduction of the bending rolls decreases the tension required betweenthe second and third set of bridle rolls, since the bending of the stripcontributes to straining it beyond its yield point. However, the use ofsuch bend rolls tend to leave the strip with a residual longitudinalcurvature which may require subsequent further treatment of the strip bymeans of a series of bending rolls of increasing diameter interlaced inthe manner of a roller leveller, this treatment being applied to thestrip after it leaves the apparatus proper.

In use, the speed differentials of the three sets of bridle rolls 401and 402, 403 and 404, and 405-408 are set so that the total elongationof the strip required is divided into two parts, the first part betweenthe first two sets of bridle rolls being such that the rolling action onthe strip is high enough to harden the strip sufliciently to preventformation of Liiders lines in the second stage, but not so high thatexcessive hardening of the strip takes place, or excessive slippageoccurs between the strip and the rolls 402, 410 and 403.

Various expedients may be adapted in the first stage to alter the ratioof stretching to hardening due to rolling that occurs. For example, ifthe separation between the rolls 402 and 403 is adjustable, the anglebetween the points at which the strip is nipped by roll 410 and thecentre of this roll is varied, thus altering the roll pressure set up bythe roll at the nip points. Alternatively, the Whole assembly of rolls401-404 and roll 410 may be replaced by any of the embodiments of theinvention previously described.

What we claim is:

1. A method of stretch levelling continuous metal strip comprisingpositively advancing the strip at two spaced locations at speedsdiffering by a percentage at least equal to the percentage elongationrequired to produce flatness in the strip, and between said spacedlocations passing the strip through at least one roll system in whichthe strip entraps a floating roll so as to cause said floating roll topinch said strip against further rolls at the points at which the stripwraps onto and unwraps from said floating roll, the diameter of thefloating roll being such that wrapping of the strip thereonto incombination with the tension to which it is subjected will cause plasticdeformation thereof, and that the ratio of the thickness of the strip tothe diameter of the roll does not exceed 0.04.

2. A method according to claim 1, wherein the strip is positivelyadvanced at a third downstream location at a rate corresponding to afurther predetermined elongation of the strip.

3. A method of stretch levelling continuous metal strip comprisingpassing the strip through two sets of bridle rolls advancing the stripat rates differing by a predetermined proportion, and between said setsof bridle rolls subjecting the strip to the action of further roll meansadapted plastically to flex the strip first in one direction and then inanother, said roll means including at least one roll supported in thegap between two further rolls separated by less than the diameter of thefirst roll by a loop in said strip passing between said further rollsand embracing said first roll, the latter having a diameter such thatthe strip will, both in wrapping onto and unwrapping from said roll, besubjected to bending stresses which, in combination with the tensionstresses applied as the result of the differential between the rates ofad Vance of the strip by the bridle roll sets, will subject the strip toplastic strain over the major part of its thickness, the ratio ofthickness of the strip to the diameter of this roll not exceeding 0.04,and the diflerence between the rates at which the strip is advancedthrough the bridle roll sets is set at a level below which Liiderslines, excessive hardening, or marks due to roll slippage occur on or inthe strip.

4. Apparatus for stretch levelling continuous metal strip comprising afirst series of bridle rolls, means for driving at least some of saidrolls, a second series of bridle rolls, means for driving at least someof said rolls, means maintaining a preselected speed differential of upto 5% between said first and second driving means, and a three rollsystem between said series of rolls defining a path for the strip in thecourse of which the strip passes through a gap between the last bridleroll of the first series and the middle roll of the three roll system,around the first roll of the three roll system, back through the gap andaround said middle roll, through a gap between said middle roll and thefirst roll of said second series of bridle rolls, around the third rollof the three roll system and back through said second gap, spacing ofthe various rolls being such that they are maintained in nestingrelationship by the tension in the strip to be handled, the ratio of thethickness of the strip to be handled to the diameter of said first andthird rolls of the three roll system not exceeding 0.04, whilst theradius of these rolls is such that bending of the strip to be handled tothis radius in combination with the tension to which it is subjectedwill cause plastic deformation thereof, said first and third rolls beinglocated by the strip itself and the remaining rolls being positivelylocated.

5. Apparatus according to claim 4, wherein all the rolls of the threeroll system are of substantially the same diameter.

6. Apparatus according to claim 4 comprising means for unnesting therolls of the three roll system from each other and the bridle rolls, andwithdrawing the centre roll of the three roll system between the othertwo rolls, and for returning the rolls to their normal position, wherebyto define an unobstructed path for strip during threading of theleveller and to deform the strip to follow its above defined path onreturn of the rolls to their normal position.

7. Apparatus according to claim 6, wherein the unnesting means comprisemeans permitting displacement of said other two rolls from the rollswith which they nest and from each other, and means withdrawing thecentre roll between said two rolls to a sutficient distance to permit aloop shaped path of large diameter to be 1 1 defined through which stripmay readily be constrained to pass during threading.

8. Apparatus according to claim 6 wherein means are provided to bodilymove the unnested rolls to a position where a straight threading pathmay be defined between the last bridle roll of the first series and thefirst bridle roll of the second series.

9. Apparatus according to claim 6 wherein said bridle rolls or thecomplete series of which they form part are displaceable apart to permitthe passage between them of the three roll system.

10. The modification of the apparatus of claim 4, wherein two or morethree roll systems are provided between the first and second series ofbridle rolls, the systems being separated by additional bridle ordeflector rolls.

11. Apparatus according to claim 4 in which the position of the centreroll of the or each three roll system relative to the bridle rollsabutting the system is adjustable whereby to control the rolling loadapplied to the strip.

12. Apparatus for stretch leveling continuous metal strip comprisingfirst means for positively driving the strip, second means forpositively driving the strip at a speed higher by a predeterminedproportion than said first driving means, and third means for drivingthe strip at a speed higher by a predetermined proportion than saidsecond means, and between said first and second driving means a rollsystem defining a path for the strip and comprising a floating roll andat least two other rolls adjacent said floating roll and separated byless than the diameter thereof, the strip path passing between saidother rolls and around said floating roll, the latter having a diametergreater than 25 times the thickness of the strip to be rolled and smallenough for the strip to be subject to plastic deformation when bent tothat radius under the influence of the tension to which the apparatussubjects it.

13. Apparatus according to claim 12, wherein a series 12 of smalldiameter bending rolls define a sinuous path for said strip between saidsecond and third strip driving means.

14. Apparatus for stretch levelling continuous metal strip comprisingfirst means for positively driving the strip, second means forpositively driving the strip at a speed higher by a predeterminedproportion than said first driving means, and between said driving meansa roll arrangement comprising two relatively large diameter backingrolls separated by a gap of width less than the sum of the diameters oftwo relatively small diameter floating rolls, the roll arrangementdefining a path for strip passing in a first direction over the firstbacking roll and through the gap, around the first floating roll in theopposite direction, through the gap and around the second floating rollin said first direction, and through the gap and over the second backingroll in said opposite direction, the small diameter rolls having adiameter greater than 25 times the thickness of the strip to be rolledbut small enough for strip to be subject to plastic deformation whenbent to that radius under the influence of tension set up in the stripby said driving means.

15. Apparatus according to claim 14 where steady rolls are located oneither side of the gap to contact the floating rolls via the strip,whereby to stabilize the position of the latter within the gap.

References Cited UNITED STATES PATENTS 2,332,796 10/1943 Hume 72-602,432,828 12/1947 Stone 72160 3,326,026 6/1967 Guillot 72-163 3,389,5916/1968 Moline 72-160 X 3,394,574 7/1968 Franek et a1. 72-164 X MILTON S.MEHR, Primary examiner US. Cl. X.R.

